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J Therm Anal Calorim
DOI 10.1007/s10973-017-6615-7
Thermal oxidation reaction of paraffin with O2 and N2O
under different pressures
Lin-lin Liu1 • Song-qi Hu1 • Pei-jin Liu1 • Guo-qiang He1 • Guan-feng Wu1
Received: 8 January 2017 / Accepted: 26 July 2017
Ó Akadémiai Kiadó, Budapest, Hungary 2017
Abstract Thermal reaction of paraffin with oxidizers
could provide valuable information concerning the
regression rate and combustion efficiency of paraffin-based
fuels. In this paper, thermogravimetric–differential scanning calorimetry experiments were carried out for paraffin
under O2 and N2O atmosphere, and the thermal reaction
kinetics of paraffin under different pressures of O2 and N2O
was estimated through the non-isothermal measurements
and model-free isoconversional methods. The results show
that the oxidation of paraffin under O2 and N2O atmosphere
represents the multistage reaction process; the obtained
activation energy (Ea) is much close by using different
model-free isoconversional methods, and paraffin is easier
to be oxidized under O2 because of the lower Ea and lower
reaction temperature; Pressure plays a positive effect on the
oxidation of paraffin, especially under O2 atmosphere.
Keywords Paraffin Kinetic parameters Thermal
oxidation reaction Hybrid motor fuel
Introduction
Hybrid motor offers several important advantages over the
liquid and solid rockets mainly due to the storage of solid
fuel and liquid oxidizer in separate phases, such as safety,
reliability, throttling of thrust, shutdown–restart ability,
environmental friendliness, low cost [1]. Normally, the
high molecular weight hydrocarbons, such as hydroxylterminated polybutadiene (HTPB) [2], polymethyl
methacrylate (PMMA) [3] and polythene (PE) [4], are used
as the fuels of hybrid motor; however, the low regression
rate of these fuels is the most key issue for the development
and application of hybrid motor [5].
Liquid tiny droplets with low surface tension and low
viscosity are produced at the tips of the waves during the
combustion of paraffin-based fuels which results in a
regression rate 3–4 times higher than HTPB [6–8].
Therefore, paraffin-based fuels are generally regarded as
the most promising fuels of hybrid motor. Nowadays there
are many researches about the paraffin-based fuels mainly
in the fields of regression rate, combustion efficiency,
combustion instability and mechanical behavior [9–11].
Ignition and combustion mechanisms are important issues
because they can provide valuable information concerning
regression rate and the combustion efficiency of fuels.
Considering that O2 and N2O are the mostly used oxidizers in hybrid motor and paraffin is the most important
constituent of paraffin-based fuels, TG and DSC experiments were carried out for paraffin under O2 and N2O
atmosphere in this study. And then, the thermal reaction
kinetics of paraffin under different pressures of O2 and N2O
were estimated through the non-isothermal measurements
and model-free isoconversional methods.
Experimental
& Lin-lin Liu
lll@nwpu.edu.cn
1
Science and Technology on Combustion, Internal Flow and
Thermal-Structure Laboratory, Northwestern Polytechnical
University, Xi’an 710072, People’s Republic of China
Semi-refined paraffin with a melting point having a range
of 60–80 °C was purchased from Daqing Petrochemical
Company, China Petroleum Corporation, and the main
composition is C22H46 1.9%, C23H48 6.4%, C24H50 12.4%,
C25H52 16.3%, C26H54 16.9%, C27H56 14.8%, C28H58
123
L. Liu et al.
11.9%, C29H60 8.8%, C30H62 4.8%, C31H64 2.9%, C32H66
1.6%, C33H68 0.5%. Simultaneous TG–DSC (METTLER
TOLEDO TG/DSC 1) and high-pressure DSC (METTLER
TOLEDO DSC 827e) were used to investigate the thermal
reaction process of paraffin with O2 and N2O under
atmospheric pressure, 1, 2 and 4 MPa. Samples weighing
about 3 mg were heated from 30 to 700 °C with a heating
rate of 10, 20, 30 and 40 °C min-1, respectively, and the
flow rate of the sweeping gas is set as 30 mL min-1. In
order to obtain some information concerning the decomposition process of paraffin, simultaneous TG–DSC
experiment under argon atmosphere was also carried out in
this study.
Results and discussions
Thermal decomposition of paraffin
TG/DSC experimental results of paraffin at argon atmosphere with 10 °C min-1 heating rate are shown in Fig. 1.
Figure 1 shows that there was an obvious endothermic
peak with peak temperature located at 70.5 °C, and this
was resulting from the melt of solid paraffin. The mass of
the sample started decrease when the temperature reached
about 210 °C with negligible endothermic effect. The mass
loss speeded up at about 241 °C, and the mass loss stopped
at 354 °C with mass loss of 99.7%. In addition, the mass
loss was accompanied with the weak endothermic process.
Paraffin consists mainly of alkane which is easily to
decompose with the products of shorter-chain-length
paraffin and olefin under higher temperature, and the produced shorter-chain-length paraffin will decompose again
[12]. The products with lower molecular weight could
escape from the sample which would result in the mass
loss. Meanwhile, the decomposition of paraffin needs
energy, so there was a weak endothermic decomposition
peak in DSC curve.
Thermal reaction of paraffin with O2 and N2O
Paraffin-based fuels react with oxidizer in hybrid motor,
and Fig. 2 shows the TG and DSC curves of paraffin at O2
and N2O atmosphere with 10 °C min-1 heating rate.
It can be seen from Fig. 2 that the mass loss started at
178 °C under both atmosphere which is lower than the
thermal decomposition temperature (210 °C) of paraffin
shown in Fig. 1. This means that the gaseous oxidizer
could react with liquid paraffin directly at lower temperature and produced small amount of gaseous products. The
mass loss speeded up at 205 °C under N2O atmosphere and
at 220 °C under O2 atmosphere, respectively, and the
samples lost most of their mass during the reactions which
was similar to the cases shown in Fig. 1. The DSC curves
show that the oxidation reaction of paraffin with O2 could
release much more heat than the case with N2O, which
agrees with the thermodynamic calculation results. There
were weak exothermic peaks located at 484.5 and 506.5 °C
under O2 and N2O atmosphere, respectively, which was
caused by the oxidation of carbon decomposed by paraffin.
In addition, the DSC curves represented insignificants
bimodal which may suggest the multistage of the reactions.
Oxidizers reacted with liquid paraffin directly at lower
temperature, and the reaction was much mild due to the
low reaction rate of heterogeneous reactions and the low
temperature. Gaseous hydrocarbon products were produced
from the decomposition of paraffin, and these products
would be oxidized by oxidizers which will improve the
decomposition of paraffin. Meanwhile, the high temperature also favors the reactions of liquid paraffin and oxidizer. Therefore, the reactions of paraffin and oxidizer
appeared the multistage reactions [13].
20
8
80
2
40
0
20
0.33%
70.5 °C
354 °C
0
100
200
300
–2
400
500
600
700
Temperature/°C
Fig. 1 TG and DSC curves of paraffin at argon atmosphere
123
Exo
N2O
10
Mass/%
60
Heat flow/W g–1 Endo
4
15
O2
178 °C
60
270 °C
40
5
484.5 °C
20
0
506.2 °C
70.5 °C
2.02%
0
–5
1.37%
100
200
300
400
500
600
Heat flow/W g–1 Endo
6
Exo
210 °C
80
Mass/%
255 °C
100
100
700
Temperature/°C
Fig. 2 TG and DSC curves of paraffin at O2 and N2O atmosphere
Thermal oxidation reaction of paraffin with O2 and N2O under different pressures
Considering that the first exothermic peak was much
obscure and little reliable information could be obtained
even after the peak fit procedure, single peak was regarded
when the DSC data were used to get the thermal reaction
kinetics parameters.
Thermal reaction kinetics
The reaction rate of condensed phase chemical reactions
usually depends on the temperature T and the reactant
conversion percentage a, and is expressed by [14]
ð1Þ
The function f(a) is the kinetic model which may take a
large number of mathematical forms depending on the
physical mechanism. For the non-isothermal kinetic analysis, k(T) which represents the temperature dependence of
the rate constant is commonly described with the following
Arrhenius equation:
E
kðTÞ ¼ A exp ð2Þ
RT
where A is the pre-exponential factor, E is the activation
energy, and R is the gas constant. For non-isothermal
experiments which are carried out with linear heating rates
b = dT/dt, the reaction rate is expressed as
da A
E
¼ exp f ðaÞ
ð3Þ
dT b
RT
The kinetic analysis based on model-free methods allows the
apparent activation energy to be evaluated for different constant
extents of conversion without assuming any particular form of
the reaction model. In this study, model-free isoconversional
Flynn–Wall–Ozawa (FWO) method [15, 16], Kissinger–Akahira–Sunose (KAS) method [17, 18] and Starink method [19]
were used to evaluate the apparent activation energy Ea of the
reactions. The three methods could be expressed by Eqs. 4–6.
Heating rate/
°C min-1
Peak temperature/°C
O2 atmosphere
N2O atmosphere
1/
atm
1/
MPa
4/
MPa
1/
atm
1/
MPa
4/
MPa
10
250
239
230
271
262
257
20
260
249
241
279
270
266
30
266
256
247
284
275
271
40
271
260
251
287
279
275
The fitted straight lines with Y (Y was set as lg b, lnb/
T1.8 and lnb/T1.8 when FWO, KAS and Starink methods
were used) as vertical axis and with 1/T as horizontal axis
are shown in Fig. 3.
The fitting report suggests that the correlation coefficients of the slope were all more than 0.99, which indicates
the high reliability of DSC data and model-free isoconversional methods used in this study. Table 2 represents the
Ea obtained by the methods mentioned above.
Table 2 shows that the Ea obtained from different
methods is much close under the same atmosphere which
indicates the isoconversional-method-free characteristics of
the reactions to a large degree, so the average value could
be used in the analysis. Table 2 also suggests that the Ea is
much lower under O2 atmosphere which means paraffin is
easier to react with O2, and this result corresponds to the
lower reaction temperature of them.
In addition, Ea decreases from 149 to 136 kJ mol-1
when the pressure increases from 1 atm to 4 MPa under O2
atmosphere, and Ea decreases from 208 to 178 kJ mol-1
for the cases under N2O atmosphere. And then, pressure
2.0
FWO-O2-1 atm
FWO-O2-1 MPa
FWO-O2-4 MPa
FWO-N2O-1 atm
FWO-N2O-1 MPa
FWO-N2O-4 MPa
1.5
AEa
Ea
2:315 0:4567
lg b ¼ lg
RgðaÞ
RT
ð4Þ
b
AR
Ea
ln 2 ¼ ln
T
Ea gðaÞ RT
ð5Þ
b
Ea
þ constant
ln 1:8 ¼ 1:0037
T
RT
ð6Þ
1.0
Y
da
¼ kðTÞ f ðaÞ
dT
Table 1 Peak temperature information of the reactions
–8
According to Eqs. 4–6, plotting lg b, lnb/T2 and lnb/T1.8
against 1/T, respectively, should give straight lines, and
apparent activation energy Ea is proportional to the activation energy the slope of lines. Table 1 shows the peak
temperature Tp of DSC curves for the thermal reaction of
paraffin under different atmosphere and pressure, which
was used to obtain Ea.
–10
1.80
1.85
1.90
1.95
2.00
1000/T/K–1
Fig. 3 The fitted curves: the solid symbols represented the data
obtained by using FWO method, and the hollow and cross-insidehollow symbols represented the ones by using KAS and Starink
methods under the corresponding conditions
123
L. Liu et al.
Table 2 The fitted Ea of thermal reactions
Method
Activation energy Ea/kJ mol
O2 atmosphere
-1
N2O atmosphere
1/atm
1/MPa
4/MPa
1/atm
1/MPa
4/MPa
FWO
150
141
137
207
191
178
KAS
148
139
135
208
192
178
Starink
149
140
136
209
193
178
plays a positive effect on the oxidation of paraffin, especially under O2 atmosphere.
Conclusions
(1)
(2)
(3)
Almost all the paraffin sample could be decomposed
into gaseous products at the temperature range of
210–354 °C, but the oxidation of paraffin starts at
205 and 220 °C under O2 and N2O atmosphere,
respectively, which indicates the multistage of the
oxidation reaction process.
The results of apparent activation energy Ea are
much close by using different model-free isoconversional methods, and paraffin is easier to react with
O2 because of the lower Ea and lower reaction
temperature.
Ea decreases with pressure indicating pressure plays
a positive effect on the oxidation of paraffin, and this
effect is more obvious under O2 atmosphere.
Acknowledgements This work is supported by the National Natural
Science Foundation of China (Grant No. 51606157) and the Fundamental Research Funds for the Central Universities (Grant No.
3102017zy007).
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